1. Field of the Invention
The present invention relates to an optical component including a diffraction grating element, an optical device including the optical component, and an optical communications system including the optical device.
2. Related Background Art
A diffraction grating element comprises a transparent flat plate, and a diffraction grating formed on one surface of the flat plate or formed within the flat plate in parallel with the one surface (see, for example, Kashiko Kodate “Development of Diffractive Optics and Future Challenges”, Bulletin of the Japan Women's University, Department of Science, Vol. 10, pp. 7–24, (2002)). In the diffraction grating element, light incident on the diffraction grating is diffracted by the diffraction grating. The diffraction angle of the light in this case differs in accordance with the wavelength of light. In other words, when light with a wavelength λ is introduced into a diffraction grating of grating period Λ at an incident angle of θ0, then the emission angle θ1 of the mth-order diffracted light, emitted from the diffraction grating, is expressed by the following Formula (1), and it differs in accordance with the wavelength λ.                               θ          1                =                              sin                          -              1                                ⁡                      (                                          sin                ⁢                                                                  ⁢                                  θ                  0                                            +                                                m                  ⁢                                                                          ⁢                  λ                                                                      n                    0                                    ⁢                  Λ                                                      )                                              (        1        )            Here n0 is the refractive index of the material surrounding the diffraction grating element.
In this way, thus diffraction grating element can be used as an optical demultiplexer for demultiplexing the incident light. Additionally, the diffraction grating element, in the case that the light is introduced in an opposite direction to that described above, can be used as an optical multiplexer for multiplexing components of the incident light. Moreover, by combining a diffraction grating element and another optical element, for example, it is possible to obtain a dispersion adjuster for adjusting the group delay time of light in accordance with the wavelength thereof. Consequently, diffraction grating elements are one of important optical components in WDM (Wavelength Division Multiplexing) optical communications systems, which transmit the multiplexed signal light of multiple wavelengths.
Furthermore, in thus diffraction grating element, the greater the absolute value of the angular dispersion Dg (the wavelength dependence of the diffraction angle θ1), then the more desirable it is in terms of the capacity to perform light multiplexing or demultiplexing readily. Here, the angular dispersion Dg is expressed by the following Formula (2).                               D          g                =                                            ∂                              θ                1                                                    ∂              λ                                =                      m                                          n                0                            ⁢              Λ              ⁢                                                          ⁢              cos              ⁢                                                          ⁢                              θ                1                                                                        (        2        )            